The sequel is already assured of box-office success, so now's the time to start thinking about what's gonna be in volume 3. Here are a few models that Levitt and Dubner could consider, in no particular order:

Paul Krugman: Used to be an equal-opportunity offender, wrote an earlier book slamming industrial-policy-style Democrats and took gratuitous swipes at liberal icons such as John Kennneth Galbraith. In recent years has shifted to a partisan line and pretty much only criticizes Democrats for not being partisan or Keynesian enough. But he's kept his credibility by tying his opinions to his acknowledged expertise on macroeconomics. Being a micro-econ, incentive-empahsizing version of Krugman seems like a real possibility for Levitt. Maybe he could start at Slate magazine and work his way up from there. Levitt's location at the University of Chicago could help him to this end--if he's short of op-ed-worthy topics, he could talk with his colleagues in the economics and sociology departments to get ideas. But first he's gotta decide whether he has policy objectives he'd like to push, or if he'd rather just focus on telling interesting stories.

Malcolm Gladwell: Careful to spread his targets. Generally takes a liberal line but isn't averse to the occasional contrarian stance. Keeps one step ahead of the curve: willing to be make mistakes, he's also likely to be interesting. When he's on top of his game, Gladwell doesn't follow the zeitgeist, he is the zeitgeist. I don't know that Levitt could become a Gladwell even if he wanted do, and, well, if Dubner could be a Gladwell, he wouldn't have needed Levitt.

Tyler Cowen: Freakonomics without the statistics. Interesting and fun speculation. Lack of data analysis paradoxically makes him less vulnerable to attack: he's only claiming to be interesting, not to be necessarily correct. Has an ideological stance but mixes things up a bit; you don't always know what's coming next. Cowen's distinguishing feature: he doesn't just have an opinion on everything, he has thoughts on everything. If Levitt wants to go in this direction, he'll have to take blogging a lot more seriously. Posting every day isn't enough; he also needs to be willing to seriously engage with others in open debate.

Greg Mankiw: A unabashed partisan. With a clear low-taxes stance, he can make his points without needing to be cute or do the 'I used to be a liberal until...' gambit. Not a bad way to go at all if you have strong convictions that you're willing to stand by. The go-to guy for reasoned conservative economic arguments. For Levitt to go this route, I think he'd want to set up a base in one of his research areas and go from there. For example, he could become an advocate for community policing, or charter schools, or whatever. (I don't think it's enough to pick a lightweight issue such as legalization of drugs; it's gotta be something that closer to the center of political debate. And, no, becoming a climate-change activist won't work; to become a Mankiw, Levitt has to write about something that he (Levitt) is an expert on.)

Steven Landsburg: Tyler Cowen's style without the thoughtfulness. Good writer but clearly takes his positions ahead of time and then follows up with whatever arguments are at hand. Warning to Levitt and Dubner: this is a path you really don't want to take.

Tim Harford: What Steven Landsburg might have been were he not so ideological. Not really an option for Levitt/Dubner, though: Harford is the sort of person who might write about Levitt's work; it would be a strange step for Levitt to move from original research to explication. I mean, sure, people do it all the time, but I think it would destroy Levitt's brand for him to shift from 'brilliant rogue economist' to 'excellent economics journalist.' On the other hand, this is the past of least resistance as suggested by the direction taken in many of the chapters of Freakonomics 2. Again, the difficulty is that it's hard to see why Levitt would want to do this, and it's hard to see that Dubner could do it on his own.

E. O. WIlson: Another path for a public intellectual. While making occasional forays into hot-button issues (if not actual punditry), he's covered his back by doing universally-respected scientific work. In many ways, this is the route that Levitt has already been taking. He might consider dropping the Freakonomics blog, not because it's taking too much of his time--I suspect he's a better organizer of his days than I am--but because the silly things on the blog might very well be diluting his well-deserved high reputation as a scholar. Really, he could just spend the next 10 years of his career following up on stuff he's already done and he'd be fine.

James Heckman: Lots of controversy, but all about academic matters and mostly below the waterline. Blows his top all the time in private settings but is much more careful in his public pronouncements. Levitt could possibly follow this path if he were to focus more on deepening some of his major research efforts.

Why do I keep writing about this? Some of it is the fascination of watching a slow-motion train wreck. And, as usual, blogging has the irresistible attraction that it takes the place of working, a similar activity that requires much more effort. Beyond this, much of my interest in following Levitt, or Mankiw, or Krugman, is that their career paths are so similar to mine. They represent alternative paths for my own life. To put it another way, when quantitative social scientists get in trouble, well, that hits close to home.

Another way to say this is that controversy sells books. And with controversy you're likely to piss somebody off. It's extremely rare to be a Stephen Hawking and sell lots of books while remaining universally respected in your field. You're more likely to be a Stephen J. Gould and be hated by some. In that sense, Levitt and Dubner got incredibly lucky the first time round, with all the money and the fame and little of the backlash. But it's hard to keep that sort of thing going forever.